Secondary cells or rechargeable batteries have been widely utilized in power tools, cellular phones, and many other portable electrical devices. In order to charge the cells such as nickel-cadmium batteries and nickel-hydrogen batteries, it has been proposed a constant-current battery charger which supplies a relatively uniform current to the cells. Normally, the constant-charging repeats a cycle of monitoring the charge current being supplied to the cells, comparing the monitored charge current with a target current, and varying the charge current at a uniform charge rate to regulate the current in match with the target current. Therefore, when charging almost exhausted cells, a large number of the above cycles have to be repeated until the charge current comes close to the target current.
The charge rate is selected to a relatively low in order to avoid undesired overshooting or undershooting of the charge current after the charge current comes close to the target current. Accordingly, the above constant-charging scheme is, in principle, difficult to make a rapid increase of the charge current towards the target current when there is a considerable difference therebetween.
Japanese Patent Publication No. 8-336236 may be interpreted to give a solution to make a rapid charging by relying upon a constant-voltage charging instead of the constant-current charging while the charge current is far from the target current. However, since a charge current may be deviated largely from the target current during the constant-voltage charging, there remains a problem that the target current is not expected to be rapidly reached as long as the constant-voltage charging is made, which eventually prolongs a time for reaching the target current.